Public telephone networks usually utilize modems which transmit data at a rate of 56 kbps, for instance. In the telephone network the modems use the same frequency range as the telephones to transmit speech, the range being about 400 Hz to 4 kHz. However, the line between a switching centre and a user, i.e. the subscriber line, allows that much higher frequencies than 0 to 4 kHz and thus more data can be transmitted. This limitation to 4 kHz is due to filters which are located in switching centres and filter the additional frequencies from a signal supplied to the backbone network. Actually, in the subscriber line, typically made of a twisted copper pair, frequencies as high as several MHz can be transmitted. xDSL technologies (x Digital Subscriber Line, such as DSL, HDSL, SDSL, ADSL and VDSL) also utilize a frequency range above the usual speech frequency. By utilizing the entire frequency range, it is possible to achieve high transmission rates.
FIG. 1 illustrates the structure of an xDSL system. xDSL technologies require a special type of modem 1 for a user, and a corresponding modem 2 in a switching centre, for example, in which case the xDSL connection (such as VDSL) over a subscriber line 3 between subscriber equipment, such as a telephone 5 or a data terminal 6, and a telephone network 4 is established by means of these devices 1 and 2. The XDSL modems 1 and 2 comprise a transceiver unit, which converts a signal, such as an ATM signal, supplied to it into an xDSL signal to be transmitted to the subscriber line 3 and, on the other hand, separates the actual signal to be transferred from the xDSL signal received from the subscriber line 3. The transceiver unit can be implemented in the form of an integrated circuit, for instance.
One of the latest xDSL technologies is VDSL (Very high-speed Digital Subscriber Line), which is now being standardized at ETSI (European Telecommunications Standards Institute). In the VDSL standard proposal, two frequency bands are allocated for both directions of transmission: D1 and D2 for downstream and U1 and U2 for upstream, as shown in FIG. 2. Thus, there are four bands altogether. They can be understood as two pairs D1+U1 and D2+U2, both comprising one band in both directions of transmission. Also ANSI (American National Standards Institute) and ITU-T (International Telecommunication Union, Telecommunications sector) have put forth corresponding four-band VDSL standardization proposals. Band pairs can be named as an upper band pair (D2+U2) and a lower band pair (D1+U1). To implement the band pairs and thus the VDSL transceiver unit (VTU), two transmitters and two receivers are required when e.g. QAM technology (Quadrature Amplitude Modulation) is used and two groups of transmitters and receivers are required when DMT technology (Discrete Multi-Tone) is used. DMT technology employs several sub-carriers within a predetermined band. In the following, these entities of separate transmitter-receiver pairs or pairs of a transmitter group and receiver group, which are included in the transceiver unit and use a certain band pair, are called transceiver blocks, and thus the VDSL transceiver unit typically comprises two separate transceiver blocks, one using the band pair D1+U1, for example, and the other using the band pair D2+U2. FIG. 3 illustrates the structure of a VDSL transceiver unit 7. In addition, the figure shows an analogue front end 8, by means of which the transceiver unit 7 is connected to the subscriber line 3. The transceiver unit comprises two transceiver blocks 71 and 72, one using the band pair D1+U1 and the other using the band pair D2+U2. In addition, the figure shows blocks 73 and 74 illustrating functions common to the transceiver blocks. An apparatus utilizing the unit 7 uses input and output interfaces IN and OUT, and so the user sees only one ingoing and one outcoming data flow. The unit 7 is connected to the subscriber line 3 via transmission and reception interfaces tx and rx. Both transmission bands U1 and U2 pass through the transmission interface tx. Correspondingly, both reception bands D1 and D2 pass through the reception interface rx.
If the distance between the user and the switching centre is very long, the subscriber line is provided with repeaters, which amplify signals passing through the subscriber line in both directions. The transmission capacity (2 Mbit/s) of current, widely used PCM repeaters (Pulse Code Modulation) can be multiplied by replacing them by VDSL repeaters. Typically, the distance between PCM repeaters in the first leap is 800 m and in the next leaps 1500 m. PCM repeaters are often located in constructions controlled by the teleoperator, such as in cable manholes under streets, telephone poles etc. If PCM repeaters are replaced by VDSL repeaters, it is easiest to keep the locations of the repeaters the same as they were. Thus, also the distances between VDSL repeaters are typically 800 or 1500 m. FIG. 4 illustrates the use of repeaters on a subscriber line. In the figure, the subscriber line between modems 1 and 2 is divided into three parts 31, 32 and 33, between which there are repeaters 41 and 42. In practice, the number of repeaters on the subscriber line is arbitrary and may thus differ from the figure.
To implement a VDSL repeater, basically two transceiver units are needed; one for each repeater direction. FIG. 5 shows an obvious implementation of a VDSL repeater which utilizes two transceiver units 7A and 7B and analogue front ends 8A and 8B thereof. The transceiver units 7A and 7B correspond to the unit 7 shown in FIG. 3. The illustrated arrangement provides both parts 3A and 3B of the subscriber line with two band pairs, such as D1+U1 and D2+U2, and thus acts as a repeater between the parts.
As the frequency used in data transmission increases, signal attenuation increases, and at high frequencies the attenuation is very strong. Naturally, the attenuation, for its part, causes that the longer the line is, the weaker the signal becomes at its other end, which is why the achieved data transmission rate is lower as well.
For example, the lower (D1 +U1) band pair of the VDSL system can utilize a longer transmission link, and even a repeater distance of 1500 m typically operates well. Depending on the distance, it is possible to achieve a data transmission rate of about 6 to 10 Mbit/s symmetrically. The upper band pair (D2+U2) increases the rate at short distances, but at a repeater distance of 1500 m in particular, it is of very little significance, and thus the upper band pair is not very significant in the repeater use, but the functionality mainly depends on the lower band pair.
A problem in the above arrangement is that in a data transmission system, such as VDSL, which utilizes several transmission and reception bands, the structure of the repeater becomes too complicated with respect to the achieved advantage when a long transmission distance is employed: the transceiver blocks allocated to the upper band pair of the transceiver units are often unnecessary in the repeater use, because, due to signal attenuation, not much data passes through them.